1. Field of the Invention
The present invention generally relates to lead-on-chip (LOC) packaging, also known as A-wire packaging, for semiconductor devices and, more particularly, to a planar leadframe having a notched lead-finger configuration for reducing mechanical stress at the chip edge.
2. Description of the Prior Art
The size and nature of modern semiconductor chips are such that it is physically impractical to incorporate both an integrated circuit and its leads in single chip structure. Hence, modem chips are manufactured without leads and are later fitted into a leadframe package structure which supplies outside leads to the chip. Early semiconductor leadframes used lead contacts which bonded directly to the input/output ports on the active face of the chip. U.S. Pat. Nos. 4,210,926 to Hacke and 4,209,355 to Burns are both examples of such directly bonded semiconductor leadframes. As illustrated in FIGS. 1A-B, a chip 12 is positioned in the leadframe 10 and surrounded by a plurality lead-fingers 14. Raised contacts 16 at the tips of lead-fingers 14 are solder bonded directly to bonding pads around the peripheral edges of the chip 12. Chips packaged in this manner are sometimes referred to as spiders since the lead-fingers resemble spider legs emanating from around a chip body. The contacts 16 are formed by partially etching the lead-fingers 14 near the tip to leave a bump of conductive material. The lead-fingers are bonded directly to the chip by applying pressure and forcing the lead-fingers into the active chip face as shown by arrow 18. The raised contacts 16 ensures that the whole finger is not smashed into the chip face when bonding pressure is applied. U.S. Pat. No. 4,924,292 to Kaufman shows a similar direct bonding technique except that instead of an etched bump, the finger is crimped near the tip to arch the length portion of the finger away from the chip surface.
As semiconductor chips become more and more densely integrated, the electrical leads required to connect the chip to the outside world become so numerous in comparison to chip size that directly connected leads are not practical. Additionally, modern semiconductor leadframe packages must also be designed to dissipate heat that large scale integrated chips generate during the course of normal operation. Several leadframe packaging technologies have been developed to address these problems. For example, FIG. 2 shows a leadframe package having lead-fingers 30 which are spaced around the periphery of the chip 32. However, rather than the finger contacting the chip directly, each finger is wirebonded to the chip. This makes it possible to connect many more leads to the chip than was previously possible with direct bonding. Wirebonds 34 electrically connect lead contacts 38 to chip contacts 36 on the periphery of the chip 32. Unfortunately, the wirebonds inherently possess high impedance which, when compounded by their length, present undesirable electrical characteristics and signal delays. Furthermore, almost no heat dissipation is provided for the chip and the peripherally distributed contacts provide multiple entry points for contamination.
U.S. Pat. No. 4,862,245 to Pashby et al. shows a Lead-On-Chip (LOC) semiconductor package, designed to eliminate the aforementioned problems. As shown in FIG. 3, all electrical chip connections route through vias 40 extending in lines down the center of the active chip face 42. Flat lead-fingers 44 are secured on either side of the chip with an insulating adhesive strip 45. Rows of lead-fingers run parallel to each other and arch directly over the active face of the chip terminating just short of the center at the adhesive strip 45. Since all connections are made at the center of the chip, shorter wirebonds 46 which are substantially uniform in length may be used. LOC packages of this type having center line connections greatly reduce the signal delays and timing variances associated with the periphery connections of the prior art and generally improve the electrical characteristics of the package. Furthermore, the lead-fingers aid heat dissipation to some degree since they tend ;to carry heat away from the chip at the adhesive strip 45.
To minimize thermal shear-stress at the chip/lead-finger interface, leadframes are normally made of a material which has a low coefficient of thermal expansion close to that of silicon, such as, for example, copper or nickel-iron alloys. However, even if the coefficients are closely matched, high stresses are still experienced in LOC packages. The larger the surface area coverage between the lead-finger and the chip face, the greater the stress becomes. Even small variations in thermal expansions tend to aggregate at the chip edge and cause cracking or other damage to the active chip face leading premature chip failure or spurious chip operation.
To minimize the thermal shear-stress problem, one technique has been to raise or arch the otherwise straight fingers over the edge of the chip such that the fingers do not contact the chip edge. As illustrated in FIG. 3 the fingers 44 arch away from the chip 42 after the adhesive strip 45. This technique is known in the art as "downsetting" and is shown in, for example, U.S. Pat. No. 5,068,712 to Murakami et al. While downsetting reduces thermal shear-stress, it suffers the drawback of making the leadframe non-planar, fragile and unstackable. This is a significant problem, particularly during the manufacturing process. For example, non-planar leadframes are not well suited for following a manufacturing track or automated assembly line. Additionally, non-planar, downset leadframes are not stackable since lead frames near the bottom may collapse under the weight of the leadframes on top. Furthermore, downset leadframes only offer minimal heat dissipation since the lead-fingers only contact a small surface of the chip face.